Why did I burn out this LED?

Thread Starter

MikeA

Joined Jan 20, 2013
254
I had 2 red LEDs connected in series to a power supply that was current limited to 300mA. Each LED is rated at 700mA and about 2.9V forward voltage. The LEDs were pulsed at about 1% duty cycle, so they were staying cool. I turned up the voltage on the power supply to see how they would work connected to a battery that provides 24V.

Once I got to 24V, the LEDs stopped working. It appears the the LED closest to the + rail burned out. The other LED is fine.

What happened here? I thought LEDs were current devices.
 

Delta Prime

Joined Nov 15, 2019
998
Hello there :) At a 70% duty cycle, an LED’s brightness should be near 70%. The correlation between duty cycle and brightness is not 100% linear, as the efficiency of LEDs varies with the amount of current supplied
 

michael8

Joined Jan 11, 2015
137
You have a power supply which puts out pulses? (my crystal ball isn't powered today).

The power supply is current limited, but perhaps there are some capacitors after the current limit so
during the pulse the current can the power supply current limit?

It appears the the LED closest to the + rail burned out.

Nice phrase, I picture a little flame bursting out of the LED along with some smoke. The 'burned out"
I take as your deduction (not saying it's wrong), just it's not an observation.

Some observations come to mind which might have happened (but I have no way of knowing);
  • now it doesn't conduct current?
  • now it doesn't light?
  • I can see a broken bond wire inside it?
  • it conducts current but doesn't light?
 

Thread Starter

MikeA

Joined Jan 20, 2013
254
but perhaps there are some capacitors after the current limit
This is exactly what is happening. It is a switching bench power supply with ~700uF on the outputs. I put an o-scope across an LED and sure enough there is a current spike at each pulse. That's what did it in. :(
 

BobTPH

Joined Jun 5, 2013
3,308
Also, current limiting is a circuit that responds to the current drawn, it cannot act instantly. By pulsing the output, you have guaranteed that it will exceed the current limit each time the pulse starts before the limiting circuit can react.

Bob
 

bertus

Joined Apr 5, 2008
21,364
Hello,

The capacitors on the output will produce a large current peak before the current limit is reached.
Use a special chip that is designed for led driving.

Bertus
 

Thread Starter

MikeA

Joined Jan 20, 2013
254
Also, current limiting is a circuit that responds to the current drawn, it cannot act instantly. By pulsing the output, you have guaranteed that it will exceed the current limit each time the pulse starts before the limiting circuit can react.
Limiting circuit response is additive to whatever the output capacitors have stored?

I know this supply is cheap (RD6006), are all power supplies plagued by delays? Is it a topology flaw? Or they all have a delay, just some are slower some are quicker?
 

crutschow

Joined Mar 14, 2008
27,198
If you had the power supply voltage on at 12V and then connected the LEDs, then the current surge from the supply output capacitor could have zapped the LED.
 

BobaMosfet

Joined Jul 1, 2009
1,773
I had 2 red LEDs connected in series to a power supply that was current limited to 300mA. Each LED is rated at 700mA and about 2.9V forward voltage. The LEDs were pulsed at about 1% duty cycle, so they were staying cool. I turned up the voltage on the power supply to see how they would work connected to a battery that provides 24V.

Once I got to 24V, the LEDs stopped working. It appears the the LED closest to the + rail burned out. The other LED is fine.

What happened here? I thought LEDs were current devices.
First of all, your understanding of the ratings for the LED, and what the manufacture really wants them run at are 2 different things. If you will provide part/identification information, we might be able to find a datasheet and explain how to read it so you understand what the LED can actually be run at.

Current and Voltage are about power (i.e. Watts). That's Watt's Law. Too much wattage means the junction in the LED fries because it simply cannot handle the heat. In some cases, if you want to run at maximum, you may have to add heatsinking to the leads (which the manufacturer will explain in the datasheet according to derating, and dissipation).
 

Thread Starter

MikeA

Joined Jan 20, 2013
254
Did you try to connect limit resistor.Maybe 220Ω
Is there a formula to calculate the resistor to sink the current spike?

Say I have an 1A current limited source of 24V, an LED with 3V forward voltage rated at 0.5A max and a current spike of 3A for a few millisecond when the LED is pulsed.
 

bertus

Joined Apr 5, 2008
21,364
Hello,

Do you have the specifications/datasheet of the used led?
In there the peak current should be given.

Bertus
 

Ylli

Joined Nov 13, 2015
1,051
To be absolutely safe, simply take the power supply voltage, subtract the expected LED drop, and divide by the max desired current. e.g.
12 volts supply, 700 mA LED with an expected drop of 2.9 volts:
(12 - 2.9)/0.7 = 13 ohms.

If you wanted to push things further, you would need to know the peak current capability of the LED, how long the LED can accept the peak current, and the size of the power supply output capacitor. Now add in heat calculations, and it becomes a little bit messier.
 
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